linux-kernel - Re: RCU Kconfig help text

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Message-ID: <20090905200145.GB7181@linux.vnet.ibm.com>
Date:	Sat, 5 Sep 2009 13:01:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Valdis.Kletnieks@...edu
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: RCU Kconfig help text

On Sat, Sep 05, 2009 at 09:27:10AM -0400, Valdis.Kletnieks@...edu wrote:
> init/Kconfig says:
> 
> "choice
>         prompt "RCU Implementation"
>         default TREE_RCU
> 
> config TREE_RCU
>         bool "Tree-based hierarchical RCU"
>         help
>           This option selects the RCU implementation that is
>           designed for very large SMP system with hundreds or
>           thousands of CPUs.  It also scales down nicely to
>           smaller systems.
> 
> config TREE_PREEMPT_RCU
>         bool "Preemptable tree-based hierarchical RCU"
>         depends on PREEMPT
>         help
>           This option selects the RCU implementation that is
>           designed for very large SMP systems with hundreds or
>           thousands of CPUs, but for which real-time response
>           is also required.
> 
> endchoice"
> 
> This leaves somebody who has a laptop wondering which choice is best for
> a system with only one or two cores that has CONFIG_PREEMPT defined. One
> choice says it scales down nicely, the other explicitly has a 'depends on
> PREEMPT' attached to it...
> 
> (Yes, I realize in practice, the RCU sections on a laptop are probably usually
> so short they don't matter in practice.  I finally concluded TREE_PREEMPT was
> apparently a rename of CONFIG_PREEMPT_RCU and went with that since that was
> working for me before...)

Good point -- I will add the "It also scales down nicely to smaller
systems" to TREE_PREEMPT_RCU.

For -really- small systems, TINY_RCU will hopefully be there at some
point, but it can only handle single-CPU systems.

							Thanx, Paul
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